A phosphor layer consisting of ZnS or ZnSe and containing a small amount of an activator such as Mn or Cu emits light when it is sandwiched between opposed electrodes (one of which is transparent) and a certain voltage is applied between the electrodes. A surface-area light source utilizing this phenomenon known as "electroluminescence" is called an electroluminescent panel.
Electroluminescent panels are classified as dispersive type or thin film type according to the manner of formation of the phosphor layer. Further, they are classified as DC type or AC type according to the manner in which the panel is excitated.
The aforementioned dispersive type is produced by dispersing fine powder of ZnS or ZnSe containing a small quantity of Mn or Cu in a solution of an organic binder to form a paste, and then applying the paste to a transparent electrode by screen printing, with a doctor knife, or by other means to form a phosphor layer. The thin film type is produced by forming a phosphor layer making use of a thin film formation technique such as evaporation or sputtering. The above-mentioned DC type and AC type are devices which are driven by a DC power supply and an AC power supply, respectively. The present invention pertains to the dispersive type electroluminescent panel.
Referring now to FIG. 1, there is shown a conventional dispersive, DC type electroluminescent panel in cross section. This panel includes a transparent base plate 1 made of glass or similar material, a transparent electrode 2 formed on the plate 1, and a phosphor layer 3 formed on the electrode by painting operation. Another electrode 4 which is opposed to the electrode 2 with the layer 3 therebetween is formed into a thin film from a metal such as aluminum by evaporation or sputtering.
When a DC voltage is applied between the electrodes 2 and 4, a large current flows across them initially, but light emission does not occur. As the voltage is increased gradually without changing any other parameter, the current produced reduces, and when the voltage exceeds a certain value, light emission takes place. This process is known as forming. After the occurrence of forming, the device emits light whose color is characteristic of the activator contained in the device, while consuming a minute electrical current.
Unfortunately, this dispersive type electroluminescent panel has the following disadvantages. The junction where the phosphor layer 3 and the electrode 4 of the panel are in contact with each other is shown in FIG. 2 in enlarged cross section. In many cases the electrode 4 does not sufficiently conform to the surface unevenness of the phosphor layer 3. The layer 3 of the dispersive type panel is formed by applying phosphor powder paste and drying it, as mentioned above, and therefore the air bubbles within the paste and formation of larger particles due to flocculation of the particles of the phosphor 5 make the surface considerably uneven. Meanwhile, since the electrode 4 is a metal thin film formed by evaporation or other means, it lacks flexibility and adhesion, producing a number of gaps 6 between the phosphor layer 3 and the electrode 4. This reduces the area of contact between the layer 3 and the electrode 4, so that the resistance between the electrodes 2 and 4 increases, thus increasing the forming voltage. The result is that the panel generates much heat during the forming, increasing the temperature of the transparent electrode 1.degree. to 100.degree. C. or more. For this reason, when the transparent plate 1 is made of a flexible synthetic resin film, the plate 1 will deform. Further, cracks will be produced in the electrode 2 formed on the plate 1, thereby breaking the electric wires. In the worst case, the panel itself may be burned. As such, the material that can be used for the transparent plate 1 of this kind of electroluminescent panel is only glass or the like and so it is impossible to obtain a flexible electroluminescent panel. In addition, the voltage at the end of the forming is high, requiring a high excitation voltage. Furthermore, if there exist gaps 6 as observed above, the corresponding portions fail to emit light, leading to a decrease in the luminance.
In an attempt to cause the electrode which is opposite to the transparent electrode to adhere to the phosphor layer, it has heretofore been proposed to introduce an adhesive layer made from a conductive resin between them, the resin consisting of a hot melt resin to which conductive fine particles such as carbon are added. Conductive resin adhesives contain a large portion of thermoplastic synthetic resin, or binder component, in the adhesive layer to increase the adhesion. Conductive fine particles are mainly linked together, resulting in electrical conductive property. The resin penetrates into the gaps in the chain-like structure. Consequently, the layer of the conductive resin adhesive exhibits a high electrical resistance ranging from hundreds of ohms to thousands of ohms, though the material is termed a "conductive" resin. The high sheet resistance prevents the forming from proceeding uniformly and creates a difference in the forming velocity between the edge portion of the light emitting surface and the central portion, thus frequently causing uneven emission of light. Thus, it is difficult to obtain a homogeneous emission of light over a large area. Additionally, a high voltage is needed to drive the device.
When the layer of the conductive resin adhesive is stuck to the phosphor layer, the phosphor layer is heated to soften and fuse it, but the high viscosity of the adhesive prevents it from penetrating into minute gaps in the surface of the phosphor layer, whereby contributing to uneven emission of light.